The invention relates principally to the separation of aqueous/organic mixtures by pervaporation. The separation is carried out using a hybrid process combining pervaporation and reflux condensation, also known as dephlegmation.
Liquids containing organic compounds and water occur throughout industry.
Such liquids include wastewater streams contaminated with dissolved organic compounds, for example methanol, ethanol or other alcohols, methyl ethyl ketone, other ketones and aldehydes, esters, such as ethyl acetate, phenols, aromatic compounds such as benzene or toluene, and other hydrocarbons, including halogenated hydrocarbons, such as trichloroethane. These organics make the water unfit for reuse or discharge, and are difficult to remove, even at low concentrations.
Other representative liquids include process streams from many chemical processes, such as those conducted in solution, and raw product streams that require purification. Examples within this area include many streams produced by the food industry, such as those that arise during preparation of beverages, such as juices, wine or beer, or when flavors or aromas are extracted. Other examples are streams generated during pharmaceutical production.
Another significant class of processes that yield organic/water containing liquid streams is fermentation. Several important materials, including acetone and ethanol, are made by fermenting corn or other biomass feedstock. Bioethanol is the most important liquid fuel made in the United States from domestically produced renewable resources such as corn and other agricultural crops and food processing wastes.
At present, the U.S. agriculture industry provides approximately 1.3 billion gallons of fuel ethanol per year. Traditional production of bioethanol involves batch fermentation of biomass into alcohol using a biocatalyst, followed by ethanol recovery from the fermentation broth using distillation. The distillation step of this energy-intensive production process accounts for about 40% of the total energy needed for corn-to-ethanol conversion. The high cost of the distillation step has discouraged increased use of this process that is otherwise very attractive to U.S. industry.
Mixtures of organic compounds and water in the vapor phase are also found.
Pervaporation is an energy-efficient membrane-separation process that is used as an alternative to distillation for removal and/or recovery of volatile organic compounds from aqueous solutions and for dehydration of industrial solvents or other organic liquids. The process can provide very selective separation of hydrophobic organic compounds, such as aromatic hydrocarbons or chlorinated solvents, from water, but is much less effective is separating more hydrophilic organics, such as alcohols and ketones.
Membrane distillation is a term sometimes used to describe a distillation process in which the gas and liquid phases are separated by a porous membrane, the pores of which are not wetted by the liquid phase.
Vapor separation is a membrane separation process in which a feed stream that is normally liquid under ambient temperature and pressure conditions is supplied to the feed side of the membrane as a vapor. Thus the process is normally performed at elevated temperatures.
The invention is a process for separating a liquid containing an organic compound and water, using a combination of pervaporation and reflux condensation, also known as dephlegmation.
The combination process can treat aqueous streams containing one or more dissolved organic compounds, to produce a product stream containing as much as 90 wt % or more organic compound. This high concentration of organic can be achieved even when the organic compound is present at relatively low concentrations in the feed, such as 5 wt % or less, and when the organic compounds are poorly separated from water by conventional organic-selective pervaporation, as is the case with alcohols, for example. In this case, membranes that provide a separation factor in favor of the organic component(s) are preferably used in the pervaporation step to produce an organic-enriched permeate, which is then sent to the dephlegmator for separation by partial condensation. The dephlegmator produces an overhead vapor rich in the more volatile component (usually the organic compound or compounds) and a bottom condensate product rich in the less volatile component (usually the water).
The combination process can also be used to dehydrate organic liquids in which water is dissolved, to yield an organic product containing as little as 1 wt % water or less, and a relatively clean water stream. In this case, dehydration membranes are preferably used in the pervaporation step to produce an organic-enriched residue product stream, from which most of the water has been removed, and a water-enriched permeate. The permeate is sent to the dephlegmator for separation by partial condensation. The dephlegmator produces an overhead vapor rich in the more volatile component (usually the organic compound or compounds, which can optionally be recirculated to the pervaporation step to increase recovery of the organic product), and a bottom condensate product rich in the less volatile component (usually the water).
The process has a number of advantageous features. For example, in a conventional pervaporation process, the permeate vapor is often fully condensed (except for any inert gases that may be present), so that the purity of the product depends entirely on the separation capability of the pervaporation step. Even if partial condensation is used, the vapor and liquid phases leave the heat exchanger together, at equilibrium, so the separation obtained depends only on the vapor/liquid equilibrium ratio at the condensation conditions. In contrast, the present invention uses a dephlegmator, from which the condensate leaves at the bottom and the uncondensed vapor leaves at the top. The dephlegmator tubes, fins or packing elements behave as wetted walls in which the up-flowing vapor and down-flowing condensate are in countercurrent contact. This provides a separation improved, for example, four-fold or six-fold compared with that provided by a simple partial condensation.
Further, only the vapor condensing at the top of the column must be cooled to the lowest temperature. In contrast, a conventional condenser requires all of the vapor to be cooled to the lowest temperature. Therefore, the cooling load required to operate the process of the invention can be significantly less than that required to operate a conventional partial condenser.
The process of the invention involves running a liquid feedstream, containing at least one organic component and water, through a membrane pervaporation system.
The pervaporation system may contain one or more membrane modules, of similar or dissimilar type, and may be arranged in any desired configuration, such as one-stage, multistep or multistage, all of which are known in the membrane separation arts.
The membranes may be chosen to provide an overall pervaporation separation factor in favor of the organic component(s) over water, or a separation factor in favor of water over the organic component(s), and may be of any type capable of operating in pervaporation mode to provide separation between organic components and water. Suitable membranes include, but are not limited to, polymeric membranes and inorganic membranes.
Transport through the membrane is induced by maintaining the vapor pressure on the permeate side of the membrane lower than the vapor pressure of the feed liquid. This is usually, but not necessarily, achieved by operating at below atmospheric pressure on the permeate side. A partial vacuum on the permeate side of the membrane may be obtained simply by relying on the pressure drop that occurs as a result of the cooling and condensation that takes place in the dephlegmator, or may be augmented by use of a vacuum pump. The vapor pressure of the feed liquid may also be raised by heating the feed solution.
The dephlegmator may be of any type capable of providing countercurrent contact between upward flowing vapor and downward flowing condensate, and to provide heat exchange over at least part of the length of the dephlegmator between the feed under treatment and an appropriate coolant. Examples of suitable types of dephlegmator include shell-and-tube and brazed aluminum plate-fin designs, as well as packed columns of various configurations.
The dephlegmation step may be carried out using a single dephlegmator, or may incorporate multiple dephlegmators arranged in series, optionally in such a configuration as to enable multiple products of different compositions to be withdrawn.
The process is useful in diverse circumstances where a solution of organic-in-water or water-in-organic is to be separated. Representative, but non-limiting, application areas are recovery of fermentation products and dehydration of organic liquids. The process may be used, for example, to yield enhanced performance in pervaporation applications, such as those in which the condensed permeate from the pervaporation unit forms a single phase, and/or is not highly enriched in one component, and/or is to be subjected to further treatment, such as distillation.
In some cases, the process of the invention can be used upstream or downstream of a distillation column to unload or simplify the distillation step, or to obviate the need for distillation entirely.
One specific exemplary area in which the process is particularly useful is bioethanol production. By incorporating the process into the production train to provide continuous removal of ethanol, the size of the fermentor can be reduced and the distillation step can be substantially reduced in size or completely eliminated.
Another specific exemplary use is to recover mixed flavor essences from evaporator condensate waters produced when fruit and vegetable juice concentrates are prepared.
In another aspect, other types of membrane separation processes capable of producing a vapor phase aqueous/organic mixture as feed to the dephlegmation step may be substituted for the pervaporation step. Suitable processes include membrane distillation, where the feed to the membrane separation step is in the liquid phase, and vapor separation, where the feed to the membrane separation step is in the gas phase.
Other objects and advantages of the invention will be apparent from the description of the invention to those of ordinary skill in the art.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.